Low torque automatic screwdriver

ABSTRACT

A pneumatic powered screwdriver adapted for moderately high speed operation and automatic shutoff at relative low torque turning resistance. An axially disengagable clutch unit intermediately couples the motor drive to the output drive bit. Contained in the clutch unit is a sear or cutoff member which is cam shifted radially in response to clutch disengagement for enabling air supply interruption to the motor.

United States Patent 11 1 Eckman et al.

[ Oct. 23, 1973 [54] LOW TORQUE AUTOMATIC 3,275,116 9/1966 Martin 173 12 x SCREWDRIVER 3,082,742 3/1963 Vilmerding et a1 173/12 X [75] Inventors: Ri char=ll E. Eckipan lames C. Primary Examiner Emest R Purser M'tche botho HoustonTex' Attorney-Robert W. Mayer, Thomas P. Hubbard, Jr., Assignee: Dresser Industries, Inc., Dallas, Tex. Daniel Rubin, Raymond j y Van kle, William E. Johnson, Jr. and Eddie E. Scott [22] Flledz June 9, 1971 [2]] Appl. No.: 151,329 ABSTRACT A pneumatic powered screwdriver adapted for moder- 52 11.5. c1. 173 12, 192/150 Speed f relative low torque turning resistance. An axially [51] Int. Cl. B25b 23/14 [58] Field of Search l73/l2 192/150 disengagable clutch umt 1ntermed1ately couples the motor drive to the output drive bit. Contained in the [56] References Cited clutch unit is a scar or cutoff member which is cam shifted radially in response to clutch disengagement UNITED STATES PATENTS for enabling air supply interruption to the motor. 3,612,236 10/1971 Fernstrom et al. 192/150 X 18 Claims, 12 Drawing Figures 10 12 12 E 52 34 14 25 24 i I 1 1 7 44 45 9 I -28 I 87 53 1a 23 26 10s a4 83 77 54 5O PMENTED RBI 2 3 I975 SHEET 10! 3 Mm Oh m wfm E 8 mm m INVENTORSI RICHARD E. ECKMAN JAMES C. MITCHELL ATTORNEY LOW TORQUE AUTOMATIC SCREWDRIVER BACKGROUND OF THE INVENTION The field of art to which the invention pertains includes the art of driving tools having a clutch for power stop control.

Powered screwdrivers of various types and capacities are well-known and have been commercially used for many years. Popular among consumers of such tools are those specifically adapted to interrupt the power supply for automatic shutoff in contrast to those which slip or ratchet on encountering a turning resistance of preset torque value. Marketed tools exemplifying the shutoff type are disclosed in U.S. Pat. Nos. 2,964,151; 2,986,052; and 3,242,996, all of which have met with a large measure of commercial success.

While fundamentally similar in purpose, construction of these various shutofftools is sufficiently different in order to render each of them operationally suitable for their specific use applications. As might be expected, tools suitable for torque sensitivity on the order of 40 inch pounds and above are substantially bulkier and heavier than such tools suitable for delicate work requiring torque sensitivities on the order of l 1 inch pounds and below. Moreover, by virtue of their respective constructions, the higher torque tools are generally capable of high speed operation on the order of 3,000 rpm whereas the low torque tools have heretofore been subject to speed sensitivity causing them to endure low speed limitations on the order of 1,000 rpm. Where used for mass production assembly, tool speed is regarded as a significant factor in contributing toward the rate of production output. Consequently, it has long been desired to increase the speed of low torque tools, yet retain their compactness and light weight features compatible with use on delicate type work.

SUMMARY This invention relates to pneumatically powered screwdrivers of the automatic shutoff type suitable for low torque conditions of use. More specifically, the invention relates to such low torque screwdrivers capable of at least medium operating speeds on the order'of about 2,000 rpm and above while essentially retaining the compactness and light weight features previously characteristic of such tools. This is obtained in accordance herewith by means of a novel clutch-cutoff construction connecting the motor output to the drive bit. The clutch provides positive torque responsive engagement and disengagement which is substantially insensitive to speed at which the tool is operative. Without speed sensitivity, previously imposed speed limitations are overcome thereby extending the operational speed range for which such tools can be constructed.

It is therefore an object of the invention to provide a novel construction for powered screwdrivers of the automatic shutoff type.

It is a further object of the invention to provide a novel construction for an automatic shutoff powered screwdriver suitable for low torque operation but capable of higher speed operation than such similar purpose tools of the prior art.

It is a further object of the invention to provide a novel construction for a low torque powered screwdriver as in the aforementioned objects while essentially retaining the basic compactness and light weight features characteristic of such tools for delicate work suitability.

I It is a still further object of the invention to provide a novel power driven clutch mechanism for use in low torque powered screwdrivers.

' BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal view partially in section of a tool in inoperative relation incorporating the construction hereof;

FIG. 2 is a fragmentary longitudinal section view of the tool of FIG. 1 in its operative relation;

FIG. 3 is an enlarged longitudinal view partially in section of the clutch mechanism;

FIG. 4 is an enlarged longitudinal fragmentary section through the tool drive corresponding to the inoperative relation of FIG. 1;

FIG. 5 is a sectional elevation viewed substantially along the lines of 55 of FIG. 4;

FIG. 6 is a sectional elevation viewed substantially along the lines 6--6 of FIG. 4;

FIG. 7 'is an enlarged longitudinal fragmentary section through the tool drive corresponding to the operative relation of FIG. 2; I

FIG. 8 is a sectional elevation viewed substantially along the lines 8-8 of FIG. 7;

FIG. 9 is an enlarged longitudinal fragmentary section through the tool drive in its post-torqued operative relation corresponding to FIG. 2;

FIG. 10 is a sectional elevation viewed substantially along the lines 10-l0 of FIG. 9;

FIG. 11 is an enlarged isometric view of the cutoff sear as enclosed within the dashed outlined in FIG. 4; and

FIG. 12 is a partial plan view as seen from the position l2-l2 of FIG. 1.

Referring now the drawings and specifically to FIGS. 1 and 2, the tool hereof is generally comprised of an elongated longitudinal housing formed of a rear section 11 enclosing the drive components and telescopically threaded to a front section 13 enclosing the work engaging and torque responsive elements. Within the latter is included a screwdriver bit or the like 14 secured in output shaft 17 for rotationally driving a screw or other similar type fastener element 15. It is understood of course that various forms of bits can be substituted for that shown as required, including but not limited to Phillips head, Allen head, socket, etc. Tool drive is furnished by a conventional vane-type pressure fluid motor (not shown) contained in housing section 11 and operationally supplied with high pressure air received at inlet 18. Output from the motor is transmitted to screwdriver bit 14 via an intermediate clutch mechanism 19 to be described.

Tool actuation is initiated by a guide piece 22 adapted to engage screw 15 and slideably contained within the necked-down work end 23 of housing section 13. For these purposes, there is provided a key 24 which rotationally interlocks the guide piece to work end 23, but permits slideable inward movement thereof. Inward movement is opposed by coil spring 26 and is limited in both directions to the extent afforded by longitudinal key slot 25 as can be best understood by contrasting the guide piece relation in FIGS. 1 and 2. Rearward guide piece displacement permits bit 14 to engage screw '15. Since the bit shank 36 is detachably mounted via a ball detent 37 to the full depth of output shaft bore 38, further rearward movement carries with it clutch 19 and a throttle rod 40 for admitting air received at inlet 18.

For a more detailed understanding of the various operating components, attention is now also directed to FIGS. 3-8. As thereshown, air inlet 18 is contained in a connector 28 screw threaded to the rear of housing section 1 1 andfit pressure tight with respect thereto by means of an annular o-ring seal 29. Fluid pressure admitted through the connector is communicated into an enclosed chamber 30 whereat it is exposed to the back face of conically shaped throttle and shut-off valve 31. The shut-off valve cooperates with an annular seat 32 for controlling fluid flow into a chamber 33 which in turn feeds passage 34 for supplying air into a motor inlet port 35. Opening and closing of valve 31 is governed by axially slidable throttle rod 40 diametrically enlarged at 41, reduced again at 42 and enlarged again at 43. The latter end is received in axial bore 48 of output drive shaft 17 subjecting the rod to urgingin the other direction by compressed coil spring 50 in chamber 30 acting against the back face of valve 31. Air exhaust is through muffler '44 and exhaust ports 45.

The motor is connected to a planetary gear train (not shown) the output of which is rerpresented by a shaft 51 journalled in ball bearing 52. Shaft 51 includes a central axial bore 53 of hexagonal cross section in which to slidably receive a hexagonal input stub shaft 54 of clutch 19 for a rotational driving interlock there between. Clutch output, as will be described below, is through radially extending annular flange 58 integral with a rearward reduced diameter portion 59 of shaft 7 19. The latter shaft portion is further reduced at its terminal end 60 at which it is formed of polygonal cam acting cross section, preferably square, extending inward of clutch 19 for reasons as will be understood. I

Clutch 19, for rotatable coupling to output shaft 17, includes a body 63 having a central bore 64 through which to slideably receive section 41 of throttle rod 40. A counterbore 65 receives output shaft section 59 to which it is axially coupled but rotatably free by means of a plurality of small diameter balls 66. The balls are adapted to seat between annular shaft groove 67 and corresponding annular clutch groove 68 and are supplied for that purpose through a radial bore 69 whereat they are retained by means of an encircling spiral type spring retainer 70.

To transmit drive from the motor output to shaft 17 the leftward end of clutch 19 (as viewed in the drawings) includes a cam 74 having a plurality of lobes 75. The lobes extend axially leftward facing toward shaft 17 at angularly displaced intervals radially outward of the shaft. Intermediate adjacent lobes, the cam is axially recessed at 76 to receive a hardened steel ball 77. At its circumferential ends each recess forms a rise 78 of gradual pitch effective for screw tightening and an opposite rise 79 of comparatively steeper pitch effective for reverse tool operation as for untightening of a screw or the like. Balls 77, in turn, extend axially inward of a plurality of radially open slots 82 formed in shaft flange 58 and angularly matched to the location of cam recesses 76.

For radially confining the balls while maintaining coupling engagement between cam 74 and flange 58 there is provided a cup shaped ball cage-retainer 83 constantly urged axially rearward by means of a coil spring 84. The spring is axially compressed between a follower 85 acting through thrust bearing 86 against the back side of retainer 83 and an opposite follower 87 (FIG. 1) axially slideable but rotationally secured to shaft 17 by means of opposite keys 88. By this means, so long as a screw 15 is being free run into torque position, balls 77 maintain clutch l9 rotationally coupled to shaft 17. Upon encountering a turning torque resistance at which the tool was set to operate, relative rotation begins to occur between flange 58 and cam 74. At that point each ball 77 follows its respective cam rise 78 in forcing follower 85 and bearing 86 increasingly rearwardaway from the cam in opposition to spring 84 until coupling disengagement results.

Concomitantly operative with of clutch 19 but operably independent disengagement thereof for effecting power interruption is a radially movable sear or release pin 91 (see also FIG. 11) in a radially extending cylindrical cavity of body 63. The scar is generally cylindrical, geometrically formed as to place its centroid location 100 radially displaced'from the rotational tool axis 101. To achieve centroidal displacement, the pin as it appears includes a relatively small top mass 102 and a comparative larger bottom mass 103. Likewise, as arranged, the pin includes side faces 92 and 93 and a lateral bore 94 axially parallel to tool axis 101 and of diameter sufficient when appropriately positioned to slideably pass throttle rod diameters 41 and 42. Inward of face 92 is a slot cutout 95 v-shaped at its top forming an apex 96 being urged against the side flat 61 of shaft end 60 by means of compressed coil spring 98.

On tool startup pin apex 96, by virtue of force exerted by spring 98, remains engaged with a shaft side flat 61 of shaft end 60. This places a short segment 97 of pin side face 93 backed up to the radial end face 99 of throttle rod portion 41 for precluding any leftward movement thereof. With the pin mass center 100 being eccentrically located opposite apex 96 relative to axis 101, centrifugal force generated by tool rotation acts continuously in a direction aiding spring 98 for maintaining apex 96 in the latter relation. When, however, clutch disengagement occurs on the set torque being encountered permitting rotation of shaft 17 relative to the motor output being supplied to clutch 19, the same relative rotation transmits a parallel reaction via corner 62 of shaft end 60 engaging apex 96 to shift pin 91 radially upward against spring 98 as best seen in FIGS. 9 and 10. On shifting of the pin, the axis of bore 94 is similarly shifted to free rod 40 of leftward interference and permitting axial movement leftward therethrough. With the pin thus positioned, fluid force acting on the backside of shutoff valve 31 assisted by spring 50 forces rod 40 leftward to compress spring 49 until valve 31 seats to immediately interrupt the motor drive. Spring 84 then acts to reset clutch balls 77 within their respective cam recesses 76. Valve 31 and rod 40 remain in the closed position while clutch 19 remains rearward. On subsequently removing the tool, springs 26, 49 and 98 restore the components to their original'startup relation of FIGS. 1 and 4. Fluid pressure aided by spring 50 holds valve 31 against seat 32 while spring 49 urges guide piece 22 and consequently clutch 19 leftward allowing all components to reset.

In order to preset the torque valve at which clutch disengagement is to be effected, there is provided an adjustment nut 106 as seen in FIGS. 1 and 12 screw threaded onto shaft 17. Nut 106 has a serrated radial end 107, facing toward angularly displaced pockets 108 in the back side of spring follower 87. Contained in the uppermost pocket is a hardened steel ball 109 adapted to normally seat in a serration valley 110 of the nut face. By means of a slot 112 formed in housing 13 in the vicinity of nut 106 access is provided thereto whereby a Phillips screwdriver end or the like can be inserted into an adjacent nut serration. Rotating the nut in either direction will threadably advance or withdraw the nut along shaft 17. This respectively increases compression or relaxation of spring 84 in maintaining the desired coupling force to be overcome in effecting clutch disengagement between shaft flange 58 and clutch cam 74. A rotatably displaceable spring clip 113 normally covers slot 112 and can be rotated for exposing nu't 106 to render adjustment in the manner just described.

In operation, the tool is supplied by flexible conduit or the like with high pressure air from a suitable source connected to inlet 18. On pressing guide piece 22 against a fastener such as a screw 15, the tool is automatically centered on the fastener while guide piece 22 is caused to slide axially rearward for the fastener to be engaged by screwdriver bit 14. Continued pressing engagement therebetween forces the screwdriver bit rearward carrying with it drive shaft 17, clutch 19 and the operating components secured thereto. Simultaneously, the same rearward movement likewise forces throttle valve rod 40 rearward to open valve 31 whereby fluid pressure in chamber 30, is admitted to chamber 33, passage 34 and motor port 35 for energizing the motor. With the motor operating, its output shaft 51 is caused to rotate whereby through its connection with stub shaft 54 drive is transmitted to clutch 19. By virtue of the engagement force imposed by spring 84, clutch balls 77 are restrained against the rise of cam pitch 78 thereby transmitting the driving force through flange 58 to output shaft 17 and bit 14.

On screw reaching its intended set torque, the turning resistance exerted against bit 14, while the motor continues operative, affords relative rotation between cam 74and flange 58 forcing balls 77 axially outward on cam rise 78. Axial ball movement in turn forces retainer cage 83 rearwardly in opposition to the force of spring 84. With relative rotation occurring therebetween, pin 91 is shifted by the reaction of shaft camming corner lobe 62 against pin apex 96. By the latter, pin bore 94 is re-positioned in cavity 90 until more nearly axially coincident with rod section 41 permitting unimpeded axially leftward movement of throttle rod 40. With axial throttle rod interference removed, fluid force acting against shutoff valve 31 aided by the force of spring 50 urges the valve in opposition to spring 49 to its shutoff relation against seat 32. Fluid flow to the motor is immediately interrupted as is tool rotation to complete the torquing cycle. On tool removal from screw 15 the reacting spring forces restore the various components to their startup relation for repeat of the torquing cycle. For removing a screw greater cam pitch on rise 79 as compared to rise 78 increases the torque available for that purpose and for which automatic cutoff is unnecessary.

By the above description there has been disclosed novel construction for a power screwdriver adapted for automatic shutoff on reaching a predetermined torque value at which the tool is preset to operate. The novel clutch construction in accordance herewith renders the tool readily capable of operating at low torque values without speed sensitivity in the manner of such tools of the prior art. To the contrary, by means of a sear disconnect having a centroid offset from the rotational axis, tool rotation is exploited to aid rather than impair engagement until such time as desired torque resistance is encountered. Consequently, the construction hereof achieves these results while retaining basic compactness and light weight features characteristic of such tools for delicate work suitability. At the same time, the tool instantly affords a substantially higher torque for reverse operation as when desired to remove a screw previously installed. Despite the improvements afforded hereby, these features are effected in a highly economical manner by the use of relatively simple and inexpensive components.

Since many changes could be made in the above construction and many apparently widely different mbodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall be interpreted as illustrative and not in a limiting sense.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A powered screwdriver comprising in combination:

a. an air motor having an inlet at which to receive a source of high pressure supply air;

b. a drive end for work engagement with a fastener member to be rotationally driven;

c. a clutch operative to couple the output of said motor to said drive end and operative to effectively uncouple said drive end from said motor output at an encountered fastener turning resistance corresponding to a torque of predetermined value; and

d. disconnect means operative concomitantly and in parallel with the uncoupling of said clutch to effect interruption of air supply to said motor, said disconnect means including:

1. a shutoff valve in said air inlet adapted to open and close the air supply to said motor;

2. a slidable throttle rod for operating said shutoff valve between its open and closed positions;

3. sear means movable radially relativeto the axis of tool rotation between a first position-effectively cocking said throttle rod against movement in a direction for closing said valve and a second position enabling said rod movement for closing said valve; and

4. first cam means on said drive end operably inde pendent of the operation of said clutch for shifting said sear means from said first to said second position.

2. A powered screwdriver according to claim 1 in which the centroid of said sear is displaced from the axis of tool rotation whereby rotational forces aid to maintain said sear in said first position.

3. A powered screwdriver according to claim 2 in which said sear is supported transversely intersecting the axis of said tool with its centroid definitively located to one side of said axis and there is included biasing means acting against said sear directionally of said centroid urging the sear into said first position.

4. A powered screwdriver according to claim 3 in which said cam means is rotationally supported on the axis of said tool and includes a plurality of lobes displaced at angular intervals about its periphery, at least one of said.lobes being effective in concomitantly with uncoupling of said clutch to radially shift said sear means from said first to said second positions in opposition to said biasing means.

5. A powered screwdriver according to claim 4in which said throttle rod is supported on the axis of said tool extending through an axially parallel bore in said sear means, said bore being located radially offset with respect to said tool axis while said sear means is in said first position to effect cocking of said throttle rod and is shifted to relatively approach axial coincidence when said sear means is shifted to said second position for uncocking of said throttle rod.

6. A powered screwdriver according to claim 5 in which said throttle rod includes a radial wall adjacent said sear means, and a wall segment of said sear means contiguous to said bore axially interferes with said throttle rod wall to effect said throttle rod cocking.

7. A powered screwdriver according to claim 6 including a housing supportably enclosing said motor, said drive end, said clutch and said disconnect means, and in which said drive end is axially slideable inward of said housing in response to a pressing work engagement against a fastener to be driven and said throttle rod is axially moveable in conjunction with said drive end to effect opening of said shutoff valve.

8. A powered screwdriver according to claim 7 in which said throttle rod operatively moves inwardly toward said drive end to effect closing of said shutoff valve in response to shifting of said sear means to said second position.

9. A powered screwdriver according to claim 8 including biasing means to operatively effect a driving relation between said motor, said drive end, said clutch and said disconnect means at the onset of work engagement with a fastener and to restore said driving relation on post-torqued removal from work engagement with the-fastener.

10. A powered screwdriver according to claim 1 in which said clutch comprises a first annular member having a plurality of lobes displaced at angular intervals about a radial face of said member with adjacent of said lobes defining an axial recess therebetween, a second annular member having a plurality of axial cutouts circumferentially opposite the recesses of said first member, interfitting means extending between each of said opposite recess and cutout to effect a rotational coupling relation between said first and second members, biasing means acting to urge one of said members toward the other for maintaining said coupling relation and second cam means operative to uncouple said first and second members in response to encountering of said predetermined torque value by said drive end.

1]. Apowered screwdriver according to claim It) in which said interfitting means comprise balls and said second cam means is defined by a surface pitch merging at least one common end of said recesses to its contiguous lobe. 7

12. A powered screwdriver according to claim 11 in which said first annular member'is operatively connected to said motor and said second annular member is operatively connected to said drive end.

13. A powered screwdriver according to claim 11in which the centroid of said sear is displaced from the axis of too] rotation whereby rotational forces aid to maintain said sear in said first position.,

14. A powered screwdriver according to claim 13 in which said sear is supported transversely intersecting the axis of said tool with its centroid definitively located to one side of said axis and there is included second biasing means acting against said sear directionally of said centroid urging the sear into said first position.

15. A powered screwdriver according to claim l4 in which said first cam means is rotationally supported on the axis of said tool and includes a plurality of lobes displaced at angular intervals about its periphery, at least one of said lobes being effective concomitantly with uncoupling of said clutch to radially shift said sear means from said first to said second positions in opposition to said second biasing means.

16. A powered screwdriver according to claim 15 including a housing supportably enclosing said motor, said drive end, said clutch and said disconnect means, and in which said drive end is axially slideable inward of said housing in response to a pressing work engagement against a fastener to be driven, and said throttle rod is axially moveable in conjunction with said drive end to effect opening of said shutoff valve.

17. A powered screwdriver according to claim 16 in which said throttle rod operatively moves inwardly toward said drive end to effect closing of said shutoff valve in response to shifting of said sear means to said second position.

18. A powered screwdriver according to claim 17 including biasing means to operatively effect a driving relation between said motor, said drive end, said clutch and said disconnect means at the onset of work engagement with a fastener and to restore said driving relation on post-torqued removal from work engagement with the fastener. 

1. A powered screwdriver comprising in combination: a. an air motor having an inlet at which to receive a source of high pressure supply air; b. a drive end for work engagement with a fastener member to be rotationally driven; c. a clutch operative to couple the output of said motor to said drive end and operative to effectively uncouple said drive end from said motor output at an encountered fastener turning resistance corresponding to a torque oF predetermined value; and d. disconnect means operative concomitantly and in parallel with the uncoupling of said clutch to effect interruption of air supply to said motor, said disconnect means including:
 1. a shutoff valve in said air inlet adapted to open and close the air supply to said motor;
 2. a slidable throttle rod for operating said shutoff valve between its open and closed positions;
 3. sear means movable radially relative to the axis of tool rotation between a first position effectively cocking said throttle rod against movement in a direction for closing said valve and a second position enabling said rod movement for closing said valve; and
 4. first cam means on said drive end operably independent of the operation of said clutch for shifting said sear means from said first to said second position.
 2. A powered screwdriver according to claim 1 in which the centroid of said sear is displaced from the axis of tool rotation whereby rotational forces aid to maintain said sear in said first position.
 2. a slidable throttle rod for operating said shutoff valve between its open and closed positions;
 3. sear means movable radially relative to the axis of tool rotation between a first position effectively cocking said throttle rod against movement in a direction for closing said valve and a second position enabling said rod movement for closing said valve; and
 3. A powered screwdriver according to claim 2 in which said sear is supported transversely intersecting the axis of said tool with its centroid definitively located to one side of said axis and there is included biasing means acting against said sear directionally of said centroid urging the sear into said first position.
 4. A powered screwdriver according to claim 3 in which said cam means is rotationally supported on the axis of said tool and includes a plurality of lobes displaced at angular intervals about its periphery, at least one of said lobes being effective concomitantly with uncoupling of said clutch to radially shift said sear means from said first to said second positions in opposition to said biasing means.
 4. first cam means on said drive end operably independent of the operation of said clutch for shifting said sear means from said first to said second position.
 5. A powered screwdriver according to claim 4 in which said throttle rod is supported on the axis of said tool extending through an axially parallel bore in said sear means, said bore being located radially offset with respect to said tool axis while said sear means is in said first position to effect cocking of said throttle rod and is shifted to relatively approach axial coincidence when said sear means is shifted to said second position for uncocking of said throttle rod.
 6. A powered screwdriver according to claim 5 in which said throttle rod includes a radial wall adjacent said sear means, and a wall segment of said sear means contiguous to said bore axially interferes with said throttle rod wall to effect said throttle rod cocking.
 7. A powered screwdriver according to claim 6 including a housing supportably enclosing said motor, said drive end, said clutch and said disconnect means, and in which said drive end is axially slideable inward of said housing in response to a pressing work engagement against a fastener to be driven and said throttle rod is axially moveable in conjunction with said drive end to effect opening of said shutoff valve.
 8. A powered screwdriver according to claim 7 in which said throttle rod operatively moves inwardly toward said drive end to effect closing of said shutoff valve in response to shifting of said sear means to said second position.
 9. A powered screwdriver according to claim 8 including biasing means to operatively effect a driving relation between said motor, said drive end, said clutch and said disconnect means at the onset of work engagement with a fastener and to restore said driving relation on post-torqued removal from work engagement with the fastener.
 10. A powered screwdriver according to claim 1 in which said clutch comprises a first annular member having a plurality of lobes displaced at angular intervals about a radial face of said member with adjacent of said lobes defining an axial recess therebetween, a second annular member having a plurality of axial cutouts circumferentially opposite the recesses of said first member, interfitting means extending between each of said opposite recess and cutout to effect a rotational couPling relation between said first and second members, biasing means acting to urge one of said members toward the other for maintaining said coupling relation and second cam means operative to uncouple said first and second members in response to encountering of said predetermined torque value by said drive end.
 11. A powered screwdriver according to claim 10 in which said interfitting means comprise balls and said second cam means is defined by a surface pitch merging at least one common end of said recesses to its contiguous lobe.
 12. A powered screwdriver according to claim 11 in which said first annular member is operatively connected to said motor and said second annular member is operatively connected to said drive end.
 13. A powered screwdriver according to claim 11 in which the centroid of said sear is displaced from the axis of tool rotation whereby rotational forces aid to maintain said sear in said first position.
 14. A powered screwdriver according to claim 13 in which said sear is supported transversely intersecting the axis of said tool with its centroid definitively located to one side of said axis and there is included second biasing means acting against said sear directionally of said centroid urging the sear into said first position.
 15. A powered screwdriver according to claim 14 in which said first cam means is rotationally supported on the axis of said tool and includes a plurality of lobes displaced at angular intervals about its periphery, at least one of said lobes being effective concomitantly with uncoupling of said clutch to radially shift said sear means from said first to said second positions in opposition to said second biasing means.
 16. A powered screwdriver according to claim 15 including a housing supportably enclosing said motor, said drive end, said clutch and said disconnect means, and in which said drive end is axially slideable inward of said housing in response to a pressing work engagement against a fastener to be driven, and said throttle rod is axially moveable in conjunction with said drive end to effect opening of said shutoff valve.
 17. A powered screwdriver according to claim 16 in which said throttle rod operatively moves inwardly toward said drive end to effect closing of said shutoff valve in response to shifting of said sear means to said second position.
 18. A powered screwdriver according to claim 17 including biasing means to operatively effect a driving relation between said motor, said drive end, said clutch and said disconnect means at the onset of work engagement with a fastener and to restore said driving relation on post-torqued removal from work engagement with the fastener. 